1. Field of the Invention
The present invention relates to white balance control techniques at the high sensitive image capture by an image capture apparatus such as a digital camera, a digital video camera and the like.
2. Description of the Related Art
Heretofore, in general, in the image capture apparatus such as a digital camera, a digital video camera and the like, to adjust color balance for light sources of different color temperatures, the photographed image signal is subjected to a white balance processing.
Here, an outline of the conventional white balance processing will be described.
First, the signal outputted from an image pickup device is digitalized by an A/D converter, and as shown in FIG. 2, is divided into a plurality of blocks. The signal of each block includes color signals including R (Red), G (Green), and B (Blue), and a color evaluation value of the block is calculated using the following formula (1), for example,Cx[i]=R[i]−B[i]/Y[i]×1024Cy[i]=(R[i]+B[i]−2G[i])/Y[i]×1024  (1)
(provided that Y[i]=R[i]+2G[i]+B[i])
When a color evaluation value (Cx[i], Cy[i]) falls within the white detection range set in advance, it is determined that the block is white, so that integral values sumR, sumG, sumB of the color pixels falling within the white detection range are calculated to calculate a white balance coefficient in accordance with the following formula (2) (Japanese Patent Application Laid-Open No. 2003-235050).WBCo—R=sumY×1024/sumR WBCo—G=sumY×1024/sumG  (2)WBCo—B=sumY×1024/sumB 
(provided that sumY=(sumR+2×sumG+sumB)/4)
Meantime, the digital camera currently available is designed such that its sensitivity can be changed over among a plurality of types such as ISO100, 200, 400, 800, and 1600 to be set. The larger these values are, the higher the sensitivity is, and photographing can be attained with a fewer amount of light, so that even when a shutter speed is fast, photographing can be made under an appropriate brightness. Under the circumstances such as a room and a night scene in which brightness is not sufficient, the shutter speed becomes automatically slow, but by increasing the sensitivity, the shutter speed can be increased.
However, as the sensitivity increase, the image ends up including many noise components, and this often causes an adverse effect of the rough image. Thus, under the present circumstances, the high sensitivity photographing enters the relationship of a tradeoff with the image
Here, coming into a question in the white balance correction at the high sensitivity is the case where the sensitivity is relatively high such as ISO800, and 1600, and the case where a low color temperature light source such as tungsten and a day light light source is used as an object lighting source. In this case, when the white balance correction gains are compared, a B GAIN is applied at a higher level. As a result, a blue noise becomes conspicuous, so that the image quality remarkably deteriorates.
In Japanese Patent Application Laid-Open No. 2003-235050, a color temperature is estimated from the WB gain determined by the white balance calculation, and when the color temperature deviates from the standard range, a chroma emphasis is provided with a limiter. According to this method, while the white balance can be prevented from becoming unnatural to some extent, no consideration has been given to the relation between the white balance and sensitivity or the noise.